In researching the new .NET Async and Await keywords in .NET 4.5, I created a simple example showing how to update a progress bar and a label while an async function is running. I also included how to cancel the process while it is taking place.
To use the code below, create a Windows form and include two buttons:
Button1= start button
Button2= stop button
Also add a progress bar called
ProgressBar1 and a label called
A few things to note:
The functions use ASYNC and AWAIT "all the way down" from the initial button click to the final VB function that supports asynchronous support;
Note the use of the
cancellationtokensourceto signal that we want to cancel things;
The final asynchronous VB function we use is
WebClient.DownloadStringTaskAsync(). This is arbitrary, and is used for example purposes only.
Please let me know if you have any questions or problems with this code.
' Simple progress bar example with cancellation and label updating ' Using the ASYNC and AWAIT keywords available through .Net 4.5 ' Helpful sources include: ' - http://msdn.microsoft.com/en-us/library/hh191443.aspx ' - http://blogs.msdn.com/b/dotnet/archive/2012/06/06/async-in-4-5-enabling-progress-and-cancellation-in-async-apis.aspx Imports System.Net Imports System.Threading Public Class Form1 Dim test As WebClient = New WebClient ' This is specific to the type of async function we're doing. Yours may be different. Dim myToken As CancellationTokenSource ' This is used to notify the async function that we want to cancel it. Dim myProgress As Progress(Of Integer) ' We use this as the vehicle for reporting progress back to our GUI thread Dim Counter As Integer = 0 ' This is a variable that we use to track our progress (specific to this example) Private Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click ProgressBar1.Value = 0 Counter = 0 myToken = New CancellationTokenSource Await startProgressBar(myToken) ' Note that the signature of our startprogressbar method includes the cancellationtokensource End Sub Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click If Not myToken Is Nothing Then myToken.Cancel() ' We tell our token to cancel End If End Sub Public Async Function startProgressBar(ct As CancellationTokenSource) As Task Dim progressTarget As Action(Of Integer) ' We have to create an event handler for the function we use to report progress progressTarget = AddressOf ReportProgress ' We assign the handler to our specific function (see below) myProgress = New Progress(Of Integer)(progressTarget) ' When we initialize our progress reporter, we pass our handler to it For I = 0 To 9 If ct.IsCancellationRequested = True Then Exit For ' We then break out of the loop End If Dim myString As String = Await RetrieveString(myProgress) ' We don't do anything with this string, just so we can run our async function Next End Function Public Async Function RetrieveString(ByVal myProgress As IProgress(Of Integer)) As Task(Of String) Dim myString As String = "" If Not myProgress Is Nothing Then 'If we have a valid progress object, report our progress using it Counter = Counter + 10 myProgress.Report(Counter) End If myString = Await test.DownloadStringTaskAsync("http://www.cnn.com") ' This is a throwaway function - just something that uses async Return myString ' We are really not doing anything with the results of this function, just returning it End Function Private Sub ReportProgress(ByVal myInt As Integer) Label1.Text = "Step Number: " & myInt.ToString ProgressBar1.Value = myInt End Sub End Class